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PubMed1984

Amino acid sequence of retinal transducin at the site ADP-ribosylated by cholera toxin.

Transducin was [32P]ADP-ribosylated by cholera toxin in bovine retinal rod outer segments and then partially purified on omega-amino octyl agarose to remove other ADP-ribosylated proteins. Trypsin digestion of the ADP-ribosylated transducin and further purification using boronate-polyacrylamide beads and high performance liquid chromatography yielded a single radiolabeled tetrapeptide, Ser-Arg-Val-Lys. The ADP-ribose is linked to the guanidinium group of arginine.

Adenosine Diphosphate Ribose↗
PubMed1994

Immunoreactivities to rhodopsin and rod/cone transducin antisera in the retina, pineal complex and deep brain of the bullfrog, Rana catesbeiana.

Birds and lower vertebrates are known to have extra-retinal photoreceptors in the pineal complex and deep brain. Although the photoreceptive function of the pineal complex has been investigated well, the exact location and nature of the deep brain photoreceptors are not known. In this study we tried to localize visual pigments and signal transduction proteins immunohistochemically in the brain of bullfrogs (Rana catesbeiana). The retina, and the brain with the pineal and the frontal organ were fixed with Zamboni's fixative and/or Bouin's solution. Immunoreactivities to three antisera against bovine rhodopsin (Rh-As), alpha-subunits of bovine rod (anti-pTr alpha) and cone transducin (anti-pTc alpha) were shown in the retina, pineal, frontal organ and hypothalamus. The retina and pineal were immunopositive to both Rh-As and anti-pTr alpha, whereas the frontal organ was immunopositive to only Rh-As and the hypothalamus was immunopositive to all three antisera. The cells which were immunoreactive to Rh-As, anti-pTr alpha and anti-pTc alpha were observed in the preoptic nucleus and suprachiasmatic nucleus in the hypothalamus. The shape of these immunoreactive cells in the hypothalamus was round or spindle-like with one or two immunoreactive nerve processes most of which were perpendicular to the ventricular surface. Western blot analysis of the hypothalamus, pineal and frontal organ demonstrated immunoreactive bands molecular weight of which corresponded to those of the retina (34 kDa, 38 kDa and 41 kDa). Thus, visual pigments and transducin-like proteins seem to exist in the hypothalamus as well as the pineal complex of frogs.

Animals↗
PubMedDate not supplied

[Oligonucleotide-directed mutagenesis of inhibitory gamma-subunits of cGMP phosphodiesterase from bovine outer rod segments. A new hypothesis on mechanisms for inhibiting catalytic subunits by gamma-subunits and activation of a holoenzyme by transducin].

Two mutants of the phosphodiesterase (PDE) gamma subunit (PDE gamma) from bovine retinal rods were synthesized by sequential transcription and translation in vitro. PDE gamma mutants R24E and H79L exhibited inhibitory properties similar to those of the wild-type PDE gamma (wtPDE gamma). At the same time, affinity to the rod outer segment (ROS) membranes is lower for R24E and higher for H79L in comparison with wtPDE gamma. The transducin alpha subunit (in a complex with the GTP non-hydrolyzable analogue, GTP gamma S) activates the trypsin-treated PDE (tPDE) inhibited by wtPDE gamma weaker than tPDE inhibited by R24E and stronger than tPDE inhibited by H79L. To explain the properties of these and earlier studied PDE gamma mutants, a new hypothesis on the mechanisms of inhibition of the PDE catalytic subunit dimer (PDE alpha beta) by PDE gamma and mechanism of the PDE holoenzyme (PDE alpha beta gamma 2) activation by the transducin alpha subunit in a complex with GTP (T alpha.GTP) is proposed: 1) two sites on PDE alpha beta for the PDE gamma binding (A- and the B-site) are different in structure. Sites on PDE gamma interacting with A- and the B-sites on PDE alpha beta are also different in structure. The site on PDE gamma interacting with the B-site partially overlaps with the T alpha.GTP binding site; 2) PDE gamma bound to the B-site provides the main contribution to inhibition of the enzyme catalytic activity; 3) T alpha.GTP first interacts with the PDE gamma bound to the A-site in the PDE holoenzyme and removes this PDE gamma in a PDE gamma.(T alpha.GTP) complex. This results in a slight increase of the catalytic activity of the PDE alpha beta gamma complex remaining bound to the ROS membranes; 4) after removal of PDE gamma from the A-site, another T alpha.GTP molecule is enabled to interact with both PDE alpha beta and PDE gamma bound to the B-site on PDE alpha beta. This interaction results in the formation of a ROS membrane-bound fully catalytically active triple complex PDE alpha beta.PDE gamma.(T alpha.GTP).

3',5'-Cyclic-GMP Phosphodiesterases↗
PubMed1994

Molecular characterization of the 77-kDa echinoderm microtubule-associated protein. Homology to the beta-transducin family.

The major microtubule-associated protein (MAP) of sea urchins and several other echinoderms is a polypeptide of M(r) 77,000. The echinoderm MAP (EMAP) is abundant in embryonic and differentiated cells, as well as in mitotic and interphase microtubule arrays. To characterize the molecular structure and function of the EMAP, we isolated a full-length cDNA clone, which has one open reading frame that predicts a polypeptide of 686 amino acids with a calculated M(r) of 75,488. On the basis of charge distribution, EMAP can be divided into two distinctive domains: The NH2-terminal basic region (amino acids 1-137, pI = 10.0) and a slightly acidic, COOH-terminal region (amino acids 138-686, pI = 5.8). This charge distribution is typical of many microtubule-binding proteins, but no significant sequence homology has been detected with any known MAPs. The EMAP, however, does show significant sequence similarity with the beta-subunit of the heterotrimeric G-protein, transducin. The homology lies in a series of 10 imperfect, 43-amino acid repeats (WD-40 repeats) that have been found in many proteins of diverse functions, including beta-transducins, Drosophila Enhancer of split, the yeast STE4, CDC4, CDC20, PRP4, and Tup1 gene products, and the dTAFII80 subunit of Drosophila TFIID. The function of these repeats still remains unknown. It is possible that these repeats are involved in protein-protein interactions, perhaps with the tetratricopeptide repeat-containing protein family. Alternatively, the EMAP may be an important link between signal transduction events and a change in microtubule organization during the cell cycle.

Animals↗
PubMed1993

A site on transducin alpha-subunit of interaction with the polycationic region of cGMP phosphodiesterase inhibitory subunit.

Activation of cGMP phosphodiesterase (PDE) by the rod G-protein transducin is a key event in visual signal transduction in vertebrate photoreceptor cells. Interaction between the GTP-bound form of the alpha-subunit of transducin (alpha t*) and the PDE inhibitory gamma-subunit (P gamma) is a major component of PDE activation. The central polycationic region of P gamma, P gamma-24-45, has been implicated as one of the sites involved in alpha t*.P gamma interaction. Here we determine the site on alpha t* that interacts with P gamma-24-45 using a photo-cross-linking approach. The synthetic peptides Cys(ACM)Tyr-P gamma-24-45-Cys (where ACM indicates acetamidomethyl group) and Cys-P gamma-24-45 were labeled with 4-(N-maleimido)benzophenone at the COOH and NH2 termini, respectively, and then cross-linked to alpha t. When the photoprobe was attached to the COOH terminus of the peptide, a specific high yield cross-linked product (80%) was formed between the peptide and alpha t GTP gamma S (guanosine 5'-O-(thiotriphosphate)). A lower yield of cross-linking (35%) was seen between the peptide and alpha t GDP. The site of cross-linking between Cys(ACM)Tyr-P gamma-24-45-Cys and alpha t GTP gamma S was localized to within alpha t-306-310 using a variety of chemical and proteolytic cleavages of the cross-linked product, analysis of the fragments with SDS-polyacrylamide gel electrophoresis, and matrix-assisted laser desorption ionization mass spectrometry.

3',5'-Cyclic-GMP Phosphodiesterases↗
PubMed1993

Enhancement by phosphodiesterase subunits of the rate of GTP hydrolysis by transducin in bovine retinal rods. Essential role of the phosphodiesterase catalytic core.

Phosphodiesterase (PDE) in bovine retinal rod outer segments is activated when it forms a membrane-bound complex with the alpha-subunit of transducin loaded with GTP (T alpha*). At maximal activation, this complex contains two T alpha* and all the subunits of native PDE (PDE alpha, PDE beta, and two inhibitory PDE gamma). We observed previously (Pagès, F., Deterre, P., and Pfister, C. (1992) J. Biol. Chem. 267, 22018-22021) that the rate of GTP hydrolysis by transducin in a rod outer segment suspension is enhanced when T alpha* is bound to native PDE (PDE alpha beta gamma 2). In this article, we compare the effects of PDE species with different PDE gamma contents. We show that T alpha* hydrolyzes its GTP faster not only when bound to PDE alpha beta gamma 2, but also when bound to PDE alpha beta gamma or PDE alpha beta. Moreover, trypsin-treated PDE (PDE gamma-deprived soluble PDE) also induces an acceleration of GTP hydrolysis. On the contrary, addition of isolated PDE gamma alone does not accelerate GTP hydrolysis. The interaction between T alpha* and PDE gamma, which is essential for the activation of PDE by T alpha*, is apparently not responsible of the feedback of PDE on T alpha*. The interaction of primary importance for the acceleration of GTP hydrolysis would be that existing between T alpha* and PDE alpha beta.

Animals↗
PubMed1993

Cooperative binding of the retinal rod G-protein, transducin, to light-activated rhodopsin.

Direct measurements of the binding between light-activated rhodopsin (Rho*) and transducin, the retinal rod G-protein, revealed a strongly cooperative interaction. Cooperativity was assessed by measuring the association of 125I-labeled transducin (Gt) to Rho* in urea-stripped rod outer segment membranes at equilibrium. Analysis of 125I-Gt binding curves gave a Hill coefficient of 1.8. These data were consistent with a two-site model in which binding of the first 125I-Gt to Rho* increased the binding of the second 125I-Gt approximately 40-fold (Kd values were 80 +/- 30 and 1.9 +/- 0.7 nM, respectively). The effects of GDP on the binding were also investigated. GDP decreased the affinity between Rho* and Gt approximately 100-fold but did not decrease the degree of cooperativity. Binding curves of 125I-Gt in the presence of 1 mM GDP showed a Hill coefficient of 1.9. The data were also consistent with a two-binding site model in which binding of the first 125I-Gt increased the binding of the second 125I-Gt approximately 70-fold (Kd values were 13.7 +/- 5.4 and 0.20 +/- 0.08 microM, respectively). The Gt alpha subunit in the absence of Gt beta gamma also bound Rho* in a cooperative manner. These data implicate a role for the cooperative association of Rho* and Gt in the light activation cascade of retinal rods.

Animals↗
PubMed1993

Characterization of mutant rhodopsins responsible for autosomal dominant retinitis pigmentosa. Mutations on the cytoplasmic surface affect transducin activation.

Rhodopsin mutants responsible for autosomal dominant retinitis pigmentosa (ADRP) were prepared by site-directed mutagenesis and characterized. The aim was to evaluate ADRP mutations that occur at three locations on the cytoplasmic surface of rhodopsin: Thr-58 near the cytoplasmic border of helix A, the tetrapeptide Leu-68 to Pro-71 in the first cytoplasmic loop, and Arg-135 at the cytoplasmic border of helix C. It was hypothesized that amino acid changes at these sites would result in mutant rhodopsins with normal spectral properties but defects in their ability to interact with the rod outer segment G protein, transducin. A set of 12 mutant opsin genes was prepared. Four of the mutants were known to cause ADRP: Thr-58 replaced by Arg, a four-amino acid deletion (Leu-68/Arg-69/Thr-70/Pro-71), Arg-135 replaced by Leu, and Arg-135 replaced by Trp. Eight additional mutants were prepared to provide complementary structure-function information. The four-amino acid deletion mutant failed to bind 11-cis-retinal. However, each of the Thr-58 and Arg-135 mutants bound 11-cis-retinal to form a pigment with a visible absorbance maximum (lambda max) of 500 nm. Upon illumination, each pigment was converted to a metarhodopsin II-like spectral form (lambda max = 380 nm). However, each of these spectrally normal ADRP mutants was defective in activating guanine nucleotide exchange by transducin. These results identify a defect in the signal transduction pathway in spectrally normal mutant rhodopsins that cause ADRP.

Amino Acid Sequence↗
PubMed1998

Canine cone transducin-gamma gene and cone degeneration in the cd dog.

PURPOSE: To characterize the cDNA and the organization of the gene encoding the cone-specific gamma subunit of transducin (Tgamma c) and to examine this gene as a candidate for the recessively inherited cone photoreceptor degeneration in the cd dog. METHODS: Canine Tgamma c cDNA was cloned and sequenced. Polymerase chain reaction (PCR) was used to define the Tgamma c gene structure, northern blot analysis to examine the level of expression of Tgamma c mRNA in control and cd-affected retinas, and immunocytochemistry to determine the presence and localization of Tgamma c in normal and cd retinas. RESULTS: Immunocytochemical results showed Tgamma c localized to cone photoreceptor outer segments in the normal retina, whereas no Tgamma c immunoreactivity was observed in the cd retinas. However, the level of transcription and the primary structure of the cloned cDNA coding for the 69-amino acid protein were identical in retinas from wild-type and affected dogs. CONCLUSIONS: Although Tgamma c immunoreactivity was specifically absent in the cd dog retina, no differences were detected between normal and cd retinas in the nucleotide sequence of Tgamma c mRNA or in its synthesis. These results indicate that a mutation in the Tgamma c gene may not be causally associated with the cd dog disease. These findings suggest that possible abnormalities in posttranslational modification of Tgamma c or defective assembly of the transducin alphabetagamma complex could lead to rapid degradation of Tgamma c.

Amino Acid Sequence↗
PubMed1991

Fourier transform infrared studies of active-site-methylated rhodopsin. Implications for chromophore-protein interaction, transducin activation, and the reaction pathway.

Fourier transform infrared studies of active-site-methylated rhodopsin (ASMR) show that, as compared to unmodified rhodopsin, the photoreaction is almost unchanged up to the formation of lumirhodopsin. Especially, the deviations are much smaller than those observed for the corresponding intermediates of 13-desmethyl-rhodopsin. In metarhodopsin-I, larger alterations are present with respect to the three internal carboxyl groups. Similar deviations have been observed in meta-I of 13-desmethyl-rhodopsin. This indicates that, in agreement with our previous investigations, these carboxyl groups are located in close proximity to the chromophore. Because this latter pigment is capable, when bleached, of activating transducin, our data provide support for the earlier conclusion that deprotonation of the Schiff base is a prerequisite for transducin activation. The positions of the C = C and C - C stretching modes of the retinal suggest that the redshift observed in ASMR and its photoproducts can be explained by an increased distance of the Schiff base from the counterion(s). It is further shown that the photoreaction does not stop at metarhodopsin-I, but that this intermediate directly decays to a metarhodopsin-III-like species.

Binding Sites↗
PubMed1998

Synthesis of Enzymatically Stable Analogues of GDP for Binding Studies with Transducin, the G-Protein of the Visual Photoreceptor.

The synthesis of five enzymatically stable analogues of guanosine diphosphate (GDP) has been carried out. The pyrophosphate moiety was mimicked in turn by the malonate, the acetophosphonate, the phosphonoacetate, the methylene-bis-phosphonate, and the imidodiphosphate groups. All the compounds were prepared via the synthesis of a transient fully protected nucleoside diphosphate analogue, and the final deprotection step was achieved by catalytic hydrogenolysis. The biological properties of the compounds have been evaluated toward transducin, the G-protein of the visual photoreceptor. Three guanosine imidodiphosphate derivatives bearing a linker at different positions on the sugar and on the base were then prepared and evaluated, giving some insight into the GDP binding site of transducin.

Journal Article↗
PubMed1997

The rat D4 dopamine receptor couples to cone transducin (Galphat2) to inhibit forskolin-stimulated cAMP accumulation.

Based on its expression pattern and pharmacology, the D4 dopamine receptor may play a role in schizophrenia. Thus it is of interest to know what signaling pathways are utilized by this receptor. Previously, we showed that activation of D4 receptors in a mouse mesencephalic neuronal cell line (MN9D) inhibited forskolin-stimulated cAMP accumulation in a pertussis toxin-sensitive (Ptx-sensitive) fashion. Of the known Ptx-sensitive G-protein alpha subunits, MN9D-expressed Galphai2, GalphaoA, and GalphaoB; however, none of these coupled to the D4 receptor. Using a low stringency polymerase chain reaction cloning method, we found an additional Ptx-sensitive G-protein cone transducin (Galphat2) expressed in the MN9D cells. We also found that Galphat2 mRNA is highly expressed in rat mesencephalic tissue. To test the hypothesis that the D4 receptor couples to Galphat2, we cotransfected MN9D cells with the D4 receptor and a mutagenized Ptx-resistant Galphat2 subunit (mGalphat2). Application of the dopaminergic agonist quinpirole to cotransfected cells inhibited forskolin-stimulated cAMP accumulation in the presence or absence of Ptx. To our knowledge, this is the first report demonstrating that the D4 dopamine receptor functionally couples to a specific G-protein and that a non-opsin-like receptor can couple with a transducin subunit.

Animals↗
PubMed1985

Inhibition of hormonally regulated adenylate cyclase by the beta gamma subunit of transducin.

Transducin (T), the GTP-binding protein of the retina activates the cGMP phosphodiesterase system, and presents analogies with the proteins GS and Gi which respectively mediate adenylate cyclase activation and inhibition by hormone receptors. These proteins are all comprised of an alpha subunit carrying the GTP-binding site and a beta gamma subunit made of two peptides. The beta peptide (35 kd) appears similar in the three proteins. We demonstrate here that purified T beta gamma inhibits adenylate cyclase from human platelet membranes. This inhibition was observed when adenylate cyclase was stimulated by GTP, prostaglandin E1 (PGE1), NaF and forskolin, but not when stimulated by GTP(gamma)S. In the presence of GTP and forskolin, the T beta gamma-induced maximal inhibition was not additive with the alpha 2-receptor-induced adenylate cyclase inhibition mediated by Gi. Both inhibitions were suppressed at high Mg2+ concentrations, which as also known to dissociate T beta gamma from T alpha-GDP. This suggests that these adenylate cyclase inhibitions are due to the formation of inactive complexes of GS alpha-GDP with T beta gamma or Gi beta gamma. T beta gamma-induced inhibition did not require detergent and could be suppressed by simple washing. T beta gamma effects are dependent on its concentration rather than on its total amount. This suggests that T beta gamma can operate in solution with no integration into the membrane. Similar inhibitory effects of T beta gamma are observed on adenylate cyclase from anterior pituitary and lymphoma S49 cell lines.

Adenylyl Cyclase Inhibitors↗
PubMed1993

Tryptophan W207 in transducin T alpha is the fluorescence sensor of the G protein activation switch and is involved in the effector binding.

We have produced a recombinant transducin alpha subunit (rT alpha) in sf9 cells, using a baculovirus system. Deletion of the myristoylation site near the N-terminal increased the solubility and allowed the purification of rT alpha. When reconstituted with excess T beta gamma on retinal membrane, rT alpha displayed functional characteristics of wild-type T alpha vis à vis its coupled receptor, rhodopsin and its effector, cGMP phosphodiesterase (PDE). We further mutated a tryptophan, W207, which is conserved in all G proteins and is suspected to elicit the fluorescence change correlated to their activation upon GDP/GTP exchange or aluminofluoride (AlFx) binding. [W207F]T alpha mutant displayed high affinity receptor binding and underwent a conformational switch upon receptor-catalysed GTP gamma S binding or upon AlFx binding, but this did not elicit any fluorescence change. Thus W207 is the only fluorescence sensor of the switch. Upon the switch the mutant remained unable to activate the PDE. To characterize better its effector-activating interaction we measured the affinity of [W207F]T alpha GDP-AlFx for PDE gamma, the effector subunit that binds most tightly to T alpha. [W207F]T alpha still bound in an activation-dependent way to PDE gamma, but with a 100-fold lower affinity than rT alpha. This suggests that W207 contributes to the G protein effector binding.

3',5'-Cyclic-GMP Phosphodiesterases↗
PubMed1995

Transducin-alpha C-terminal mutations prevent activation by rhodopsin: a new assay using recombinant proteins expressed in cultured cells.

We have measured the activation by recombinant rhodopsin of the alpha-subunit (alpha 1) of retinal transducin (Gt, also recombinant) using a new assay. Cultured cells are transiently transfected with DNAs encoding opsin and the three subunits of Gt (alpha t, beta 1 and gamma 1). In the microsomes of these cells, incubated with 11-cis-retinal, light causes the rapid activation of Gt, as measured by the ability of GTP gamma S to protect alpha t fragments from proteolytic degradation. The activation of Gt is also observed when all-trans-retinal is added to microsomes under constant illumination. Activation depends on both opsin and retinal. Opsin mutants with known defects in activating Gt show similar defects in this assay. alpha t mutations that mimic the corresponding mutations in the alpha-subunit of Gs also produce qualitatively similar effects in this assay. As a first step in a strategy aimed at exploring the relationships between structure and function in the interactions of receptors with G proteins, we tested mutant alpha t proteins with alanine substituted for each of the 10 amino acids at the C-terminus, a region known to be crucial for interactions with rhodopsin. Alanine substitution at four positions moderately (K341) or severely (L344, G348, L349) impairs the susceptibility of alpha 1 to activation by rhodopsin. All four mutants retain their ability to be activated by AIF-4. Two other substitutions (N343 and F350) resulted in very mild defects, while substitutions at the remaining four positions (E342, K345, D346 and C347) had no effect. In combination with previous observations, these results constrain models of the interaction of the C-terminus of alpha t with rhodopsin.

Amino Acid Sequence↗
PubMed1996

MEKA/phosducin attenuates hydrophobicity of transducin beta gamma subunits without binding to farnesyl moiety.

Hydrophobic modifications of transducin (T) gamma, such as farnesyl-and carboxyl-methylation, are essential for the association of T beta gamma with the photoreceptor disc membrane, and MEKA/phosducin is known to inhibit the association. In this study, we examined the effect of MEKA on the hydrophobicity of T beta gamma. MEKA could bind to T beta gamma without farnesyl/carboxyl-methyl moieties as well as native T beta gamma. In the Triton X-114 phase separation assay, T beta gamma-MEKA complex was recovered in the aqueous phase, whereas T beta gamma was recover in the detergent phase. N-terminal portion of MEKA which includes T beta gamma-binding domain was not sufficient to reduce the hydrophobicity of T beta gamma or to dissociate T beta gamma from the membrane. The data suggest that MEKA attenuates the hydrophobicity of T beta gamma to result in the dissociation of T beta gamma from the membrane without directly binding to farnesyl/carboxyl-methyl moieties.

Animals↗
PubMed1996

Gene structure and chromosome localization to 7q21.3 of the human rod photoreceptor transducin gamma-subunit gene (GNGT1).

The transducin gamma-subunit gene (GNGT1) encodes a member (gamma1) of the family of heterotrimeric G-protein gamma-subunits that is specific to rod photoreceptors. In this report we have determined the complete structure of the GNGT1 gene and have localized it to human chromosome 7q21.3 using somatic cell hybrid and yeast artificial chromosome analysis.

Amino Acid Sequence↗
PubMed1995

The Gnb5 gene is a novel beta-transducin homolog transcribed from a divergent promoter located immediately upstream of the Syrian hamster p53 P1 promoter.

Regulatory regions controlling p53 gene transcription in Syrian hamster embryo cells were characterized by use of chloramphenicol acetyl-transferase (CAT) constructs encompassing various subfragments of its 5'-flanking sequences. This analysis identified a 961 bp PstI-SacI (PS) fragment upstream from the p53 P1 promoter, which exhibited promoter activity only in the reverse orientation relative to the p53 gene. Northern hybridizations of mRNA from hamster embryo cells with genomic probes containing the PS fragment detected a 2.1-kb transcript expressed at much lower levels than the p53 mRNA. Steady-state levels of the 2.1-kb mRNA were threefold higher in actively growing cells than in cells from confluent cultures. Library screenings with PS-containing probes resulted in the isolation from exponentially growing cells of a cDNA, the nucleotide sequence of which showed no significant homology to genes previously described. This novel gene, named Gnb5, for guanine nucleotide-binding protein, beta 5, codes for a protein of 538 amino acids with a highly acidic amino terminus containing a proline-rich domain, followed by a neutral domain with five repeat units of the beta-transducin (WD-40) motif. The homology with beta subunits of G proteins and with other WD-40 repeat-containing proteins was restricted to the repeats. The Gnb5 gene is well conserved in rodents and primates, as the hamster Gnb5 cDNA recognized, under high stringency conditions, the human and mouse counterparts in Southern and Northern hybridizations. Expression of Gnb5 in adult tissues was detected preferentially in testes, in both hamsters and humans.

Adult↗